The invention relates generally to a rotating electrical machine having a superconductive coil, and in particular to a system and method for quench monitoring and control of a superconductive rotor coil of a rotating electrical machine.
A superconductor is a material that will conduct electricity with no electrical resistance. Most electrical conductors have some electrical resistance. However, electrical resistance is an undesirable property for a conductor to have because the electrical resistance consumes energy as heat. Superconductivity occurs in certain materials when the material is cooled below a critical temperature.
The rotors of rotating electrical machines typically use an electrical current flowing through a coil to produce a magnetic field. In commercial power generation systems, the amount of electrical current that flows through the rotor coil may be significant. However, a portion of the electrical current is consumed as resistive heating, reducing the efficiency of the power generation system. Consequently, superconductive rotor coils have been developed for use in rotating electric machines. At ambient temperatures, the coil has a defined electrical resistance. However, when cooled below the critical temperature, the coil enters a superconducting state and loses its electrical resistance.
If the current flowing through a superconductive rotor coil is driven beyond a critical current limit, a portion of the superconductive rotor coil may change from the superconducting state to a normal resistive state. The portion of the coil in the normal resistive state will cause resistive heating to occur in the superconductive rotor coil. If the resistive heating of the coil continues, the coil may enter a state of irreversible thermal runaway, known as a quench. A sustained high current density in the coil may lead to intense local heating, causing a rapid temperature rise in the region of the coil experiencing the quench. As a result, a sufficient temperature gradient may be generated in the rotor coil to cause differential expansion to occur that may, in turn, lead to strain related damage in the coil. Quenching may occur in a superconductive rotor coil for reasons other than the coil current exceeding the critical current. Other potential causes of quenching in the superconductive coil may include a loss of cooling, a failure of the thermal insulation around the coil, bad joints at current lead junctions, and the degradation over time of the superconducting properties of the coil.
Typically, quenching in a superconductive rotor coil is detected by measuring the voltage developed across the rotor coil. However, this method of detecting quenching is problematic in generators because large inductive voltages are generated across the coil during normal operation of the generator, thus making it difficult to determine when quenching is actually occurring in the coil.
Accordingly, a technique that enables quenching to be detected in a superconductive rotor coil without using the voltage across the coil as the indicator of quenching is desirable. In addition, a technique that enables the superconductive coil to be protected from damage caused by quenching is also desirable.